So far no known trigger has been found. Across the map, in the direction of the final switch you hit prior to moving on, is a cache of Forerunner weapons: Binary, Incineration, and Light Rifle. Within the vicinity of the Xeno is a data pad....that's about it. Of the area we also know there are 3 special/secret weapons laying around and a vehicle (Reddit Link)---------------------------------------------------Mission 15; Guardians (all three weapons are found under the vision of Guardians, when you look up there will be a Guardian looking at you from one direction. Is this a hint? or just coincidence?):

White Scar [Modified Plasma Caster that detonates when in proximity of hostiles and detonates in a frenzy of needles. Comes with 10+5 Plasma Bolts when you find it. Does not use regular Plasma Caster ammunition] *Not sure if this is in the Core area.

Vorpal Talon [Variant of Energy Sword which allows longer thruster hang-time and additional thruster evade. 100% energy charge when you find it. Unable to reload] ; Knowing the ability of the Vorpal Talon, could it possibly help us clamber onto once unreachable locations? Easter egg locations maybe? *I can confirm this is in the Core area.

This is also an Ultra Ghost sitting on the ground on the right side of the Cryptum

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Prior Xeno's in Mission 2 were activated by shooting, walking up with appropriate weapon equipped, and GPing from Xeno onto an energy sword.

UPDATE 7/29/16: Still stuck in the mud. One difference noted on Co-op is that there is a dead Elite on the ledge next to the Xeno, while playing solo this Elite is below the Xeno on the ground. Gamebuster1990 indicated arrows on the pillars near each Core, investigation ongoing. It is also suggested that the naming of the Cores might have something to do with it as well.

UPDATE 8/10/16: Just to add a couple facts we know about but wasn't added to the OP. Even if you clear the area of the cores out a Grunt on a Ghost continuously respawns after a set amount of time. And added to the special/secret weapons, the Ultra Ghost. Current methods being attempted are Co-op Legendary runs involving acronyms of the core names to determine core destruction order and weapon set-ups. A few have managed to get under the map as well.

UPDATE 10/14/16: Grunt Birthday Party trigger found along with an invis-elite which triggers another time. End result is still unknown. I will update more when I have time.Invis-Elite Spawn. Credit to Macnut Crunch for triggering it and eLcHuPaNiBrE 93 for figuring out the trigger.

Melee the Intel next to the Xeno with an Energy Sword and the Grunt Birthday Party sound will trigger

Invisible Elite will spawn in at the center platform near the Xeno and run for the Ultra Ghost

Killing the Elite with the Sword will trigger another Grunt Birthday Party sound, and that is where we are now. Click on the link above to catch up.

Marine - Bronze

I'll take a look at it when I get a chance.

UPDATE 2/12/18:The next step (and possibly the entirety of the egg?) has been found, thanks to zEnzine . Co-op is recomennded due to its difficulty. The steps in their entirety as we know them are shown below:1. Locke must hit the audio log by the Xeno with an energy sword, after that, an invisible elite will spawn near the Cryptum2. The invisible elite must be killed while locke is holding an energy sword3. Destroy all the cores and enter the Cryptum4. Kill all enemies in the cryptum until you are told to go down the elevator platform. DO NOT GO DOWN THE ELEVATOR. Wait about one minute, some Knights will spawn. 5. Kill the knights, and then a second wave will spawn halfway through. When you kill the second wave of Knights, make sure Locke is holding an energy sword as the last night is killed. 6. Several massive knights will spawn, once you kill these giant knights, a wave of about 20 Knights the size of ants will spawn. Once you kill all these knights, uh, nothing else happens. That may be the end of the egg, or maybe not!

ODST - Bronze

Maybe, there are some earlier on in the mission. There's that one area right after you exit the moving platform at the beginning. Nothing seems to be there, and yet there are a bunch of cliffs you can clamber.

Marine - Gold

Awesome, I'm really enjoying trying to figure these out. Just wanted to say, make sure you keep your eyes open for knives sticking out of rocks (like from the first mission I think). I have a feeling they're going to be part of some of these other eggs too.

Marine - Onyx

ODST - Bronze

Wow. I must be really good at finding Easter Eggs. Was playing through Legendary wuth my Friend, and we were playing Unconfirmed. You know that one Soldier that grabs the SAW when you enter the Mine? So, I headshotted him with the Binary Rifle and it triggered the GBP Yay when he died. I thought I had a clip, but I don't. There is also a generator that does nothing, and can be destroyed.

Marine - Onyx

Wow. I must be really good at finding Easter Eggs. Was playing through Legendary wuth my Friend, and we were playing Unconfirmed. You know that one Soldier that grabs the SAW when you enter the Mine? So, I headshotted him with the Binary Rifle and it triggered the GBP Yay when he died. I thought I had a clip, but I don't. There is also a generator that does nothing, and can be destroyed.

Sec3OP

Wow. I must be really good at finding Easter Eggs. Was playing through Legendary wuth my Friend, and we were playing Unconfirmed. You know that one Soldier that grabs the SAW when you enter the Mine? So, I headshotted him with the Binary Rifle and it triggered the GBP Yay when he died. I thought I had a clip, but I don't. There is also a generator that does nothing, and can be destroyed.

Damn, you just happen to like Eggs I think. Try reproducing it, I'm not sure of which soldier it is you are talking about.

Marine - Gold

I was thinking that maybe the Vorpal Talon's hovering may come into play. Tried hovering at the xeno and no reaction. There don't appear to be any jumps that can only be made with the double boost or hover. I didn't see any obvious GP targets from it either. I've scanned around with the Twin Jewels, but nothing spotted yet. Has anyone tried activating in co-op? Maybe some of the eggs are solo and some co-op.

Marine - Onyx

Wow. I must be really good at finding Easter Eggs. Was playing through Legendary wuth my Friend, and we were playing Unconfirmed. You know that one Soldier that grabs the SAW when you enter the Mine? So, I headshotted him with the Binary Rifle and it triggered the GBP Yay when he died. I thought I had a clip, but I don't. There is also a generator that does nothing, and can be destroyed.

Do you have the GBP skull enabled in campaign? i.e. you never headshot a grunt at same time as hitting the soldier?

How do you hover Vorpal Talon as I've no idea how to do that.

What controller settings is everyone using for the climbing, clambering. I saw a vid somewhere that said to go further in jumps you need to RUN > JUMP > HOVER > THRUST > JUMP. My timing is rubbish just like my Halo playing so need some practice but I do agree seems a lot of climbing areas in this mission.

Marine - Gold

To hover just smart link it (while in the air). It feels like it's almost triple the hover time you get with any other weapon. It also gives you two thrusts when equipped fyi.I was wondering the same thing about the bday party skull, and maybe an 2for1 kill on soldier and a grunt.I'm not sure when/if I'll be on today but feel free to add me.

Marine - Onyx

WortWortWarthog I got a GBP YAY after taking on 2 soldiers and an elite. I did kill something on a ghost but the body despawned.

as just where you get the double power Binary rifle ( I tried to shoot a Guardian in the eyes but missed).I noticed you can have the team pick up the White Scar and Binary Rifle but they wont take the sword saying "you better keep that".

Nearly cleared out the level and now attempting some jumps to higher places.

I have also used ground pound to knock out power core things. One good hit is all it takes

ODST - Bronze

Wow. I must be really good at finding Easter Eggs. Was playing through Legendary wuth my Friend, and we were playing Unconfirmed. You know that one Soldier that grabs the SAW when you enter the Mine? So, I headshotted him with the Binary Rifle and it triggered the GBP Yay when he died. I thought I had a clip, but I don't. There is also a generator that does nothing, and can be destroyed.

Do you have the GBP skull enabled in campaign? i.e. you never headshot a grunt at same time as hitting the soldier?

How do you hover Vorpal Talon as I've no idea how to do that.

What controller settings is everyone using for the climbing, clambering. I saw a vid somewhere that said to go further in jumps you need to RUN > JUMP > HOVER > THRUST > JUMP. My timing is rubbish just like my Halo playing so need some practice but I do agree seems a lot of climbing areas in this mission.

Anyone wants to try on co-op for a while hit me up please.

Ok, this was on Legendary CO-OP with another friend. When you first enter the mine during Mission 5, you see a Soldier Captain walk up to a Weapon Cache and pick up a SAW. I killed him with a Binary Rifle during the animation, and then the GBP YaY went off. No Covenant even appear on Meridian, so that's why I noticed it.

Sec3OP

Quote:

Ok, this was on Legendary CO-OP with another friend. When you first enter the mine during Mission 5, you see a Soldier Captain walk up to a Weapon Cache and pick up a SAW. I killed him with a Binary Rifle during the animation, and then the GBP YaY went off. No Covenant even appear on Meridian, so that's why I noticed it.